LONDON (Reuters) - Europe and the IMF urged Donald Trump on Wednesday to step back from the brink of a trade war, after the resignation of his top economic adviser emboldened those encouraging him to push ahead with tariffs on imported steel and aluminum.The departure of Gary Cohn, seen as a bulwark against Trump’s economic nationalism, hit shares, oil and the dollar on Wednesday, as investors saw an increased likelihood of tit-for-tat trade measures that would depress global growth.Trump plans to impose a duty of 25 percent on steel and 10 percent on aluminum to counter cheap imports, especially from China, that he says undermine U.S. industry and jobs.